This invention relates to ionizers and more particularly, to ionizers in which electrons bombard a sample to produce secondary ions for mass spectrometry.
Mass spectrometers operate on the principle of ionization of a sample and separation of the ions according to their mass, or more precisely, their mass-to-charge (m/e) ratio. There gas sample is scanned, producing a mass spectrum, where the amplitude of each peak is proportional to the molecular concentration of each component.
Quadrupole mass spectrometers provide a number of advantages over other types of mass separation methods including rapid scanning, linear mass scale, good resolution, high sensitivity, small size and high reliability.
The quadrupole mass spectrometer consists of a ionizer, focusing lens, mass filter (quadrupole) and a detector housed in a vacuum chamber operated at very low pressure. As sample gas is introduced into the instrument, it is ionized by electrons. Each component within the sample fragments into ions of known mass. The ions are directed to the quadrupole consisting of four parallel rods in symmetrical planes. By operating the rods at the proper voltages and frequency, the quadrupole operates as a selective electronic filter permitting ions of only a particular mass to charge ratio to pass to the detector. Scanning is accomplished by varying the amplitude of the DC/RF ratio over the mass range to be scanned.
Ions that strike the detector produce signals that are amplified and presented as a spectrum of signal intensity vs. mass to charge ratio (m/e). Each component has a unique fragmentation pattern of fingerprint which permits identification. By comparing the sample signal intensity with a signal intensity from a known calibration standard, the unknown sample component concentrations are quantitatively determined. A quadrupole mass spectrometer provides process control and environmental monitoring of multiple components with faster, more accurate, continuous, multiple component measurements, as well as greater reliability and ease of operation.
Mass spectrometers require that the sample molecules be in the gas phase and that the molecules be charged. This is accomplished by an ionizer. U.S. Pat. Nos. 4,388,531 and 4,447,728 --Stafford, et al show typical quadrupole filter mass spectrometers wherein a sample is bombarded with electrons to produce ions which are accelerated through a quadrupole filter which selectively discriminates the ions according to mass.
In such ionizers, auxiliary heaters are sometimes employed at added cost and decreased reliability of operation.
Secondary electron emission is a problem, because it is desired to direct all electrons to the ionization chamber to bombard the samples.
Another problem encountered in these ionizers is contamination on the focusing and accelerating electrodes. Ions striking these electrodes eventually produce a coating on the electrodes which causes erroneous measurements.
The quadrupole filter mass spectrometer is particularly suitable for on-line control of chemical processes and environmental monitoring because of its fast measurement time and easy adaptability to a wide range of measurements. It is desirable to make the quadrupole filter mass spectrometers reliably accurate over long periods of time so that they can be used in process control.
The foregoing disadvantages of prior art analyzers are obviated in accordance with the present invention. Further, the improved ionizer of the present invention adds reliability and accuracy over long periods of time to make a quadrupole mass spectrometer suitable for process control and environmental monitoring.